1. Field of the Invention
This invention relates to a method and apparatus for separating chrominance and luminance information from a quadrature modulated composite color video signal, and more specifically application of adaptive comb filtering to a composite video color signal to improve separation of chrominance and luminance video signal components during transition states, while maintaining a "legalized" comb filter output.
2. Description of the Prior Art
Comb filtering of video signals is known; see Raby U.S. Pat. No. 5,424,784, ("Raby") incorporated herein by reference in its entirety. Composite color video signals include a brightness (luminance or Y) signal and a color subcarrier signal which is modulated in phase and amplitude to represent hue and saturation of the picture. The subcarrier signal is demodulated to produce color difference signals R-Y, B-Y, and G-Y which are combined with the Y signal for reproduction of red, blue, and green signal information. Simple filters are known to separate the chrominance (chroma) and luminance (luma) from the composite video signal in e.g. television receivers. A video comb filter performs this separation of the luma and chroma signals from the composite video wherein the luma and chroma signals have the same frequency, by using the phase relationship of the chroma signal across multiples of the horizontal scan rate (across several horizontal scan lines).
The two predominant television standards are PAL and NTSC. In PAL TV the chroma vector in a flat field of color advances by 270.degree. per line. Therefore it is known that using a straightforward comb filter architecture, the composite signal from lines which are two horizontal scan lines apart are added to cancel out the chroma vector and produce an average luma signal which can be subtracted from the middle (intervening) horizontal scan line to produce the chroma signal. In NTSC TV, the chroma vector in a flat field of color advances by 180.degree. per line. Therefore it is known to add the composite signals from two consecutive horizontal scan lines to produce an average luma signal (the chroma signal having been canceled out) which can be subtracted from one or the other of the horizontal scan lines to produce the chroma signal.
Therefore with a flat field of color, i.e. having no major picture (hue or brightness) transitions, in both PAL and NTSC TV complete separation of the luma and chroma signals is possible. No cross color (luma processed as color) or cross luma (chroma processed as luma) artifacts are present. However, it is also known that when spatial or temporal discontinuities in the picture occur, the comb filter fails, resulting in undesirable comb filter artifacts in the output picture. The most visually objectionable comb filter artifacts appear in the high frequency portion of the composite video signal, because the magnitude and phase of the high frequency signal has been modified by the discontinuity (transition) in the picture content. These changes in both magnitude and phase in the color output signal can produce color difference components that are outside the so-called RGB color cube ("illegal"), making it virtually impossible to process the decoded information and to re-encode it back into a composite signal without a noticeable loss of vertical picture content.
This process of decoding and encoding is typically undertaken for purposes of (digital) processing of the components (luma and color difference signal) of the composite signal. Such digital processing is performed in certain types of television receivers, other video processing apparatus, and in studio broadcast equipment.
In the above-mentioned simple filters, the composite video is typically fed to a low pass filter to provide the luma and to a bandpass filter centered around the subcarrier frequency to provide the chroma. In addition to other disadvantages, such simple filters are deficient in completely separating chroma and luma components. For example, in order to separate the subcarrier signal for adjusting its amplitude for color intensity control or for demodulation, it is desirable to select only the subcarrier and its sidebands without also including high frequency brightness components. Conversely, the full range brightness (Y) signal will include the color subcarrier, even if the Y range is limited, since in for instance NTSC TV the subcarrier modulation may extend below 3.58 MHz by over 1 MHz. Simply stated, there is some high frequency luma (or cross color) that appears in the spectrum of the chroma, and there is some high frequency chroma (or cross luma) that appears in the spectrum of the luma.
Thus simple filters cannot completely separate composite video into chroma and luma. The appearance of either the Y signal in the chroma or the chroma in the Y signal produces undesirable patterns and distortion of the reproduced television picture. The simple filter approach does have advantages in that the low frequency luma signal is always the correct magnitude and phase while the bandpass high frequency signal, whether luma or chroma, is likewise always the correct magnitude and phase. This ensures that if the decoded picture is subsequently encoded back into a composite video signal the resulting waveform does not contain any "illegal" colors or transitions.
A comb filter removes the cross luma from the chroma signal and the cross color from the luma signal while maintaining as much as the chroma and luma bandwidth as possible. Unlike a simple one dimensional band split filter, in "flat" areas of the picture, i.e. areas in which there are no temporal or spatial changes in the picture, two dimensional comb filters can provide perfect separation of luma and chroma signals without any loss of bandwidth or cross luma or cross color artifacts.
However it is well known that in any comb filter architecture, whether adaptive or nonadaptive (an adaptive comb filter is described in Raby), whenever the comb filter fails to separate the chroma signal from the composite signal due to spatial or temporal transitions in the picture, the magnitude and phase of the resulting luma and chroma cannot be guaranteed. When this occurs, the luma and chroma signals can generate the so-called illegal luma or demodulated color difference signals that translate into the RGB components being outside the legal RGB color cube.
Legalization (correction) of the comb filter output removes the need for clipping or color correcting the color difference signals. This minimizes any illegal colors or transitions in the composite video signal which is subsequently re-encoded from the component signals produced by the decoding process.
Two non-adaptive comb filter architectures illustrate the concept of legalization of comb filter output. FIG. 1 shows a block diagram of a two horizontal scan line delay comb filter, which separates luma and chrominance without using a PAL modifier and is suitable for PAL TV. As is well known, in a PAL modifier one can cause a one line change in the quadrature modulated chroma signal if one multiplies a given chroma signal by a sine wave at twice the subcarrier frequency of one of the quadrature components.
This comb filter averages the composite video signal across two horizontal scan line delays (i.e., using three scan lines) to cancel the chroma as described above, then subtracts the resulting luma signal from the composite video to obtain chroma. The chroma signal is band limited (bandpass filtered) and subtracted from the center tap of the comb filter to provide the luma signal. Hence as shown in FIG. 1 a digital or analog composite video signal is input at terminal 12. It is to be understood that terminal 12 (as shown) for a digital composite video signal is a bus, e.g. of 8 to 12 bits, as are the other lines shown in FIG. 1 for the digital video version. Input terminal 12 is connected to the input terminal of both adder 14 and a one horizontal scan line store (delay) 16. Line store 16 stores a pixel for a duration of one video line and conventionally includes e.g. a set of series-connected registers. The output terminal of line store 16 is connected to the input terminal of a second line store 20. The output terminal of the second line store 20 connects to the second input terminal of adder 14. Hence adder 14 adds (where three consecutive horizontal scan lines are designated A, B, and C) a pixel of line A to a pixel of line C thus canceling the chroma and providing an average luma signal across three scan lines. (It is understood that due to line stores 16, 20, all three pixels A, B, C are vertically adjacent in the picture.) The output of adder 14 is multiplied by 1/2 by multiplier 22 to average the pixels of lines A and C at the output terminal of multiplier 22. Subtractor 24 then subtracts this averaged luma value from the pixel of the center line B provided from the output terminal of line store 16.
The output signal from subtractor 24 is then bandpass filtered by bandpass filter 28 and the output signal from bandpass filter 28 is the chroma signal. The composite video signal output from line store 16 is then provided to the other input terminal of subtractor 34 and the chroma signal is subtracted from that to provide the luma signal. Hence the sum of the chroma signal at output terminal 32 and the luma signal at output terminal 36 is equal to the input composite video signal at terminal 12.
The vertical frequency characteristics of the chroma at output terminal 32 of the filter of FIG. 1 are shown in FIG. 2. As illustrated this filter has the inherent problem that the chroma output at terminal 32 has a gain of 2 shown by the vertical scale (magnitude) when the vertical information in the picture changes by 180.degree. per line period. (The small numbers shown vertically along the horizontal scale of FIG. 2 are the phase degrees change per horizontal scan line period.) The resulting luma signal, when the chroma signal has a gain of 2, has the correct magnitude but undesirably is 180.degree. out of phase and hence is referred to as an "illegal" signal both because of the incorrect chroma gain (magnitude) and the luma being out of phase.
FIG. 3 illustrates another comb filter block diagram, but having four scan lines of delay. Thus the structure of FIG. 3 has many of the elements of FIG. 1 except that, instead of single line stores 16 and 20, there are two-line stores 44 and 48. In this case the separation of the chroma and luma is achieved by averaging the composite video signal across the four line delay of line stores 44 and 48 in order to cancel the chroma, and subtracting the resultant luma signal from the center tap of the comb filter to provide the chroma signal. The chroma signal is filtered by bandpass filter 28 and subtracted from the center tap of the comb filter to provide the luma signal.
The vertical frequency characteristics of the comb filter of FIG. 3 as shown in FIG. 4 are free of the chroma gain error, i.e. the magnitude of chroma being 2.0 associated with the two line cosine comb filter of FIG. 1. However, undesirably the comb filter of FIG. 3 extends any failures in the comb filter across four horizontal scan lines and hence severely reduces the diagonal luminance resolution. Thus the filter of FIG. 3 provides better resolution than that of FIG. 1 and has a legalized output due to the maximum gain of unity. However, undesirably doubling the number of line stores increases cost and also causes any failures to be over four horizontal scan lines. That is to say, the wide aperture filter of FIG. 3 tends to smear diagonal picture information and undesirably to soften the picture vertically.
This reduction in diagonal luminance while not illustrated in FIG. 4 is shown in Table 1 in which a diagonal transition over 5 horizontal scan lines (designated lines OH to 4H) causes a wider spread of the transition and also larger and a larger numerical difference between the output terminal and the center tap (2H) of the comb filter. The values in Table 1 represent luminance on a scale of 16 to 235.
TABLE 1 ______________________________________ 4H 16 16 32 128 219 235 235 235 235 235 3H 16 16 16 32 128 219 235 235 235 235 2H 16 16 16 16 32 128 235 235 235 235 1H 16 16 16 16 16 32 128 219 235 235 0H 16 16 16 16 16 16 32 128 219 235 3 line 16 16 16 20 52 127 200 231 235 235 5 line 16 16 20 44 75 127 176 208 231 235 ______________________________________
In Table 1, each entry in the row labelled "3 line" is the weighted sum for that column of (3H/4)+(2H/2)+(1H/4). Each entry in the row labelled "5 line" is the weighted sum for that column of (5H/4)+(2H2/2)+(OH/4); video lines 3H and 1H are ignored.
In the decoder described in Raby, each of the inputs to the comb filter is also passed through a simple decoder to provide a measure of the low frequency luminance and the magnitude and phase of the high frequency portion of the composite signal. This allows signals that would cause the output of the comb filter to be larger in magnitude than a simple decoder output, and which do not have a high frequency difference in phase between inputs to the comb filter which is consistent with the chrominance signal, to control the adaptation between different comb filters. Therefore this decoder has the drawbacks of the comb filter architecture of present FIG. 1 as described above.
Thus it would be desirable to have a comb filter which overcomes the above-described deficiencies in the prior art.